I just had the same thing happen to me. For me as well, the problem was fixed by restarting. Just in case it was hardware related (like some button problems), I shut down orderly, then hit the reset button to boot it.
Here is a picture of a graybar before (top half) and after (bottom half) I had scribbled upon it (outside the pictured part) for a minute or so.
http://www.math.ntnu.no/~hanche/tmp/...000-fading.jpg
(Hmm, no inline images allowed? Well, just follow the link then.)